Efficacy of Baloxavir Against Influenza in Hospitalized Patients: The INFLUENT Study (INpatients inFLUENza Treatment)

Inclusion Criteria: * Age ≥ 18 years * Participant or participant representative capable of giving signed informed consent. * Positive reverse transcriptase-polymerase chain reaction (RT-PCR) for influenza A and/or B confirmed on arespiratory tract sample. * Patient requiring hospitalization. * National Early Warning Score 2 (NEWS2) of ≥4 at planned randomization Exclusion Criteria: * Ongoing pregnancy or breastfeeding (self-reported by the participant or diagnosed by the treating phyisician) * Known contraindication to baloxavir or to the placebo * Participant weighing \< 40 kg * Patients already on NAI therapy for the current influenza episode for \> 24 hours at the time of randomization. * Prior treatment with baloxavir for the current influenza epidose * Immunosuppression defined as 1) cancer treatment with significant negative effect on the immune system; 2) immunosuppressive therapy (treatments comprising a dose of ≥20 mg/day prednisone or equivalent when administered for ≥ 2 weeks, biological therapies, steroid sparing drugs); 3) HIV infection if CD4+ T cell count \< 500/µL; 4) organ or stem cell transplantation; 5) patients on the waiting list for a transplant * Severe underlying respiratory comorbidity requiring long-term oxygenotherapy at home. * Severe disease requiring ICU care directly at hospitalization. * Severe hepatic insufficiency or any other severe medical condition when participation in the study puts the patient at risk according to the investigator's judgment. * History of inclusion in this study during a previous influenza season * Inclusion in another interventional study with an investigational drug 30 days before inclusion in the study. * Unability to consent or patient representative unable to consent.